1 The compression index Cc is 2 The unloadreloading or recom

1. The compression index (Cc) is?

2. The unload/reloading or recompression index (Cr) is?

From the void ratio-vertical stress results from a consolidation test

Solution

For a consolidation test, the compression index is the slope of the graph after loading and before unloading.

As in the slope of the straight section in the graph above will give the compression index for the sample here.

a.) Hence, compression index = Change in e / change in log of vertical stress

Cc = (1 - 0.6)/(1000 -100) = 0.4 / 900 = 4.44 x 10^-4

b.) The recompression index for a consolidation test is given by the slope of the graph once unloading starts.

Apparently, it can be observed that the recompression index would be slope of the line drawn once the vertical effective stress starts decreasing.

Hence Cr = (0.64 - 0.6)/ 900

Cr = 4.44 x 10^-5

NOTE: The recompression as in the unloading in the graph is denoted by the decrease in the vertical stress. To put in this way, the sample at hand, basically starts retracting towards its original state, however, the retract happens to be different from the initial state. Think of a plastic to understand this. You pull the plastic apart, and once you release it, it tries to get back in shape, however, some change in shape remains.